Transcript Latching SMA Microactuator

Multilayer Microfluidics
ENMA490
Fall 2003
Brought to you by:
S. Beatty, C. Brooks, S. Dean, M. Hanna, D. Janiak, C. Kung,
J. Ni, B. Sadowski, A. Samuel, K. Thaker
Problem Definition
Motivation
– BioMEMS research is
growing rapidly, but
restricted to single layer
microfluidics
– Development of a multilayer
microfluidic design would
increase flexibility
Goal
– Design, construct, and test
a controllable microfluidic
device with at least two fluid
levels
– Identify appropriate
materials, processes, and
device geometries
Problem Scope
Design Requirements
– Two-level microfluidic network
– Active control elements
Material Requirements
– Ease of patterning and use in microfabrication
– Chemically inert
– Low Cost / Obtainable
– Optically transparent
– Specific Elastic modulus (flexible, rigid)
Constraints
– Assume external fluid control
– Neglect biochemical reactions in channels
– Keep design feasible for manufacturing
Initial Material Choices
Substrate Material
• Silicon
•
•
Relatively inexpensive
Commonly used in microelectronics
•
•
Well known properties and processing
techniques
Pyrex
•
Transparent to visible light
•
•
Allows visual monitoring of micro channels
More expensive than silicon
Initial Material Choices
Microchannel Material
• Poly(dimethylsiloxane) or PDMS
•
•
•
Inexpensive
Poor surface adhesion – releasable from mold
Highly flexible
•
•
modulus of 2.5 MPa
SU-8
•
Is a photoresist
•
•
•
High aspect ratios obtainable
Good surface adhesion to silicon and pyrex
Very rigid – complementary to PDMS
•
modulus of 4000 MPa
Defined
Problem
Project Development
Divided into research groups
(BioMEMS, Materials, Devices, and Circuits)
Developed Stage 1
(Initial Microchannel Design Concept)
Developed fluid control device to
manipulate fluid flow
Developed and tested Stage 3
(Final Design: Pressure Actuated Valve Design)
Summarized
manufacturing and
experimental results
of final design
Modified design to
integrate vertical vias
for multilevel fluid flow
Developed and tested Stage 2
(Modified Microchannel Design)
Device Design: Stage 1
(Initial Microchannel Design Concept)
• Objective
– To create an initial design
for a multilayer micro
fluidic device
I/O
Bottom layer
I/O
Middle layer
• Initial design elements
– 90o orientation of fluid
layers
– Vertical interconnects at
channel intersections
– Each layer has same
design- reduces number
of molds
– Versatility of fluid paths
Top layer
Device Design: Stage 1
(Initial Microchannel Design Concept)
Materials
– Stackable PDMS layers
– Silicon substrate
– SU-8 molds
Processes
– Create a channel mold and an interconnect mold using SU-8
– Create PDMS layers from SU-8 mold: two layers from
channel mold, one interconnect layer
– Stack layers on substrate starting with a channel layer,
interconnect layer and second channel layer at 90o
orientation
Device Design: Stage 2
(Modified Microchannel Design)
Device Objective
– To test the viability of a two-level passive micro-fluidic device
Modifications from Stage 1
– Moved reservoir positions to fit existing packaging
– Created discrete flow paths to test flow on individual layers
and between layers
– Increased all dimensions to facilitate fabrication and testing
Device Logic
– Five distinct fluid paths
– 11 I/O
– Two distinct channel
levels
– One interconnect level
– One top cover level
Reservoir (I/O)
Interconnect
Device Design: Stage 2
(Modified Microchannel Design)
Device Geometry
–
–
–
–
Chosen for process
compatibility
Rectangular microchannels
Square interconnects
Circular reservoirs
Critical Dimension Value
PDMS Layer
Height
100mm
Micro-channel
Width
500mm
Interconnect Width
1000mm
Interconnect Depth
1000mm
Reservoir Diameter
0.4 cm
Materials
–
–
SU-8 used as a mold for the PDMS layers
All PDMS layers stacked on a Silicon substrate
Device Design: Stage 2
(Modified Microchannel Design)
Process Sequence
1.
2.
Begin with four polished Si wafers
Spin SU-8 (negative photoresist) on the Si wafers and prebake at 95°C
3. Align each of the four wafers with one of four masks and
expose the SU-8 to ultraviolet light, then post-bake at 95°C
4. Develop the SU8 so that the unexposed areas are removed
– Results in four distinct SU8 molds
5. Spin PDMS on the SU8 molds less than the vertical
dimension of the SU-8 protrusions
– Mix PDMS (Sylgard 184, Dow-Corning) 10:1 with curing
agent
– Spin on PDMS
– Dip the Si wafer in a sodium dodecyl sulfate(SDS)
adhesion barrier and allow it to dry naturally
– Bake in box furnace for 2 hours at 70°C
Device Design: Stage 2
(Modified Microchannel Design)
6.
Delaminate and
stack all four PDMS
layers in the following
order: Micro-channel
Layer 1, Interconnect
Layer, Micro-channel
layer 2, Top Cover
Layer
Processing Problems
• Substantial amount of cracking in SU-8 layer
• Layer assembly problems
–
–
–
–
Razor blade/ tweezers method
Layer thickness
Wrinkles
Air pockets
• Feature alignment
– Extremely difficult
– Inaccurate
Cracks in reservoir
region of SU-8 mold
Stage 2
(Experimental Results: Trial 1)
Problems
• Thickness of PDMS
layers
• Interconnects
• Delamination
• Air bubbles
Stage 2
(Experimental Results: Trial 2)
Improvements
• Successfully made and
aligned four layers
• Layers had very few
defects
• All interconnects joined
two different layers
• Entire wafer looked very
good- no rough edges,
no air bubbles between
layers, no craters
Stage 2
(Test Results: Trial 2)
Successes
• Liquid flow in all
channels
– Completely through 2
out of 5 channels
• Tracked fluid flow
using bright food
coloring
• Tested the effects of
vertical interconnects
Problems
• No capillary action
– had to use pressure
from syringe
• Pressure caused
delamination
• Functionality of
vertical interconnects
Stage 2
(Test Procedure)
Stage 2
(Channel Layout)
Reservoir (I/O)
Interconnect
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Objective
– To integrate an active control element into a basic
microchannel design based on Stage 2
Modifications from Stage 2
– Removed all microchannels except for T-shaped
section
– Added a completely top layer microchannel
– Incorporated negative pressure gas valves in design
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Logic
–
–
–
–
–
–
Two distinct fluid paths
Five I/O
Two channel levels
One gas channel level
One thin flex layer
One top cover layer
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Geometry
–
–
–
–
Made for feasibility
4 gas control sites
1 fluid interconnect
Thin PDMS flex layer
Critical Dimension
Value
SU-8 Layer Height
100 µm
PDMS Layer Height
100 µm
PDMS Flex Layer
Height
50 µm
Micro-channel Width
500 µm
Valve Width
500 µm
Valve Length
500 µm
Materials
–
–
–
–
–
SU-8 for rigid portions in valve design (gate)
SU-8 for fluid layers
PDMS for gas control layer
PDMS used for flexible gas/fluid membrane
2 substrates required (Si, Pyrex)
Device Design: Stage 3
(Pressure Actuated Valve Design)
What we need
Deflection Equation
• Deformation between
30-60 µm
• Pressure difference
between fluid and gas
of 24 - 41.6 torr
closed
Gas
•
•
•
•
•
•
Liquid
open
w =0.0318P(ab)2 (1-)/(Et3)
P: pressure
E: elastic modulus
 : Poisson’s ratio
a & b: width and length of
membrane
w: maximum deflection
t: thickness
Device Design: Stage 3
(Pressure Actuated Valve Design)
Fluid Flow Modeling
– Assumed fluid flow rate based on fluid velocity
• Based on literature search: 1500 cm/minute = 2.5 E5
μm/sec
• Fluid flow rate: 1.25 E 10 μm3/sec = 0.0125 cm3/sec
– Used the fluid flow rate calculated to determine the
following properties for the fluid flow path:
• Fluidic resistance and pressure gradient:
R = ΔP/Q [(N*s)/m5]
• Reynolds number:
Re= (rvDh)/μ
• Velocity:
v = Q/A
• Cycle time
t = Length/v
Device Design: Stage 3
(Pressure Actuated Valve Design)
Fluid Flow Modeling Results
– R (circular cross section) =
8μL/(πr4)
• μ = fluid viscosity= 0.01
g/sec*cm
• L = Length of channel
• r = Radius of channel
– R (rectangular cross
section) ~ 12μL/(wh3)
• w = Width of the channel
• h = Height of the Channel
– Total Fluidic Resistance =
RR + RM + RI + RV
RR + RM + RI + RV
Path Region
Reservoir
Micro-channel
Interconnect
Valve
Total Path
Fluidic Resistance
(g/sec*cm4)
33
9264000
24
3000000
12264057
Pressure
Gradient (Torr)
0.00031
86.9
0.00023
28.13
115.03054
RTotal
Reynolds Velocity
Number (cm/sec) Time (sec)
4.0
0.1
20.7
41.7
25
0.15
12.5
1.3
0.016
48.1
125
0.00008
20.86608
Stage 3
(Fabrication Results)
Alternative Valve Designs
• Design Elements
– Isolated fluid chamber
– Membrane division between chamber and fluid channel
– Stopper to aid in the control of the fluid
• Phase Change Bubble Valve
– Principles of Actuation
•
•
•
•
Volatile liquid (cyclopentane)
Resistive heaters
Heater cause fluid to change from liquid to gas
Expansion from gas pressure deflects membrane
SU-8
PDMS Flex Layer
PDMS Fluid Layer
SU-8 Bottom Layer
Heater
Alternative Valve Designs
Electrolytic Bubble Valve
SU-8
– Principles of actuation
PDMS Flex Layer
• Water
• Two electrodes
• Application of current causes
electrochemical reaction
• Creation of bubbles increase
pressure in chamber
Piezoelectric Valve
– Principles of actuation
• Piezoelectric material
electrically activated
• Expansion causes
compression in liquid
chamber
• Compression translated to
membrane deformation with
larger amplitude
PDMS Fluid Layer
SU-8 Bottom Layer
Electrodes
Future Work
Design:
• Improve scaling to accommodate additional
layers
Materials:
• Replace Pyrex with acrylic as top substrate
• Promote adhesion/seal between PDMS layers
• Alter surface chemistry of channels to be
hydrophilic
Summary
• Technology for multilevel microfluidic devices
has the potential to increase design flexibility
• We succeeded in fabricating two-level
microfluidic circuits with vertical interconnects
and valves
• We experienced the design, fabrication, and
testing phases of a multistage project
• Modeling and experimental feedback are
essential to evolution of design
• We learned that project organization and
management are critical to meeting project
goals
We learned to work as a team!